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Comparison of Site Classification from V_S30, V_S10, and HVSR in Italy

Instituto Metodologie Analisi Ambientali, CNR Contrada Santa Loja, Tito Scalo (PZ) 85100, Italy gallipoli{at}imaa.cnr.it

Marco Mucciarelli

Dipartimento Strutture, Geotecnica, Geologia Applicata, Università della Basilicata, Viale dell’Ateneo Lucano 10, 85100 Potenza, Italy marco.mucciarelli{at}unibas.it

The aim of this work is to verify if (1) the average of shear-wave velocity from the surface to 30 m depth (V_S30) is a good proxy of site amplification in a country with complex geology like Italy, (2) the grouping of V_S30 in different soil classes and relevant spectra in the Italian seismic codes is adequate, and (3) shallow shear-wave profiles (V_S10) could be a more economical tool for site classification. To answer the first two questions, we examined 40 estimates of V_S30 derived from microzonation projects we performed in Italy (27 obtained with downhole measurements, plus 13 velocity profiles obtained with surface techniques). In all the sites, we installed seismic instrumentation to record earthquakes and to estimate site response using horizontal to vertical spectral ratios (HVSR). The comparison between HVSR and V_S30 showed in about one-third of the sites that V_S30 is not a good proxy of observed amplification effects if the site does not have a monotonically increasing velocity profile. The reason V_S30 does not provide satisfactory estimates in Italy is linked to peculiar geological settings that are widespread in the country. We then compared the observed amplifications from earthquake HVSR data with the ones provided by the Italian seismic code, noting a substantial underestimation by the code, a somewhat unsettling situation because HVSR is usually considered to be a lower bound for amplification estimated with other techniques.

Finally, we studied 45 V_S downhole profiles to 30 m depth performed also at sites where earthquake recordings are not available. On this data set, we noticed that V_S10 could predict site classification with the same performances of V_S30. We consider alternative soil classification schemes that include soil frequency besides the velocity profile. In this two-parameter approach, V_S10 could be substituted for V_S30.

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